tatara-ui — Nord-themed CLI UX for the tatara toolchain

Three layers, every one content-addressable:

1. Palette ([palette]) — canonical Nord values + semantic Role vocabulary mapped via RoleMap::default().
2. Sigils ([sigil]) — typed Unicode glyphs (❄ ✓ ✗ ⟡ → ● ◇ ⚡ ⚙ …) each bound to a default Role.
3. Events ([event]) — UiEvent is a typed enum that serializes to tatara-lisp Sexps. A whole run is an EventStream; BLAKE3 of the stream JSON is the run-identity hash that tatara replay <hash> uses.

Plus the cache-aware fun:

• Themes are derivations: (deftheme nord-arctic …) parses via #[derive(TataraDomain)]; ThemeSpec::id() is a BLAKE3 of the spec.
• Every artifact line shows ⚡ cached or ⚙ built 5.3s next to a ◇ blake3:cxx3i50 short hash, so cache hits tell a visual story.
• Summary banner at the end shows the content-root hash + {built, cached, failed, total} counts.

(deftheme nord-arctic
  :description "the canonical tatara look"
  :semantic    (:primary "#88C0D0"
                :success "#A3BE8C"
                :warn    "#EBCB8B"
                :error   "#BF616A"))